An electric swivel is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure. It can be used in any electromechanical system that requires unrestrained, intermittent or continuous rotation while transmitting power and/or data. High voltage (HV) swivel are used offshore in order to transfer power between a static part, usually the part moored to the seabed and a rotating part that moves with the vessel around the single point mooring.
Such a high voltage swivel is known from U.S. Pat. No. 7,137,822 in the name of the applicant. The known swivel is a high voltage swivel for offshore applications, for instance for distributing electrical power, that is generated on a weathervaning Floating Production, Storage and Offloading vessel (FPSO) which FPSO is anchored to the sea bed via a turret to a sub sea power cable.
Geostationary hydrocarbon or gas risers extend upwards from a well head to a power plant on the vessel, in which the hydrocarbons or gas are converted into electrical energy. The electrical connection of the rotating vessel to the stationary sub sea power cable leading to shore is achieved by the high voltage swivel in which the stator is connected, via the geostationary swivel part on the vessel, to the sub sea power cable and the rotor is connected to the power plant on the vessel.
The known swivel has the disadvantage that there is a risk of short circuits after the system has been in operational use for a while and the conductors start to show some wear. When debris originating from wear get in suspension in the dielectric oil or in the narrow space between the conductors and the insulating rings, short circuits can be created, causing the swivel to malfunction. Upon wear of the spring elements at the contact surfaces of the annular conductors, the solid insulator rings and conductors and the enclosure of the swivel need to be dismantled in order to obtain access to the electrodes.
Further, electrical swivel are provided with HV cables which have a very high bending radius and that require space to be bended.
In Direct Current (DC), the passage of electricity through liquids is generally accompanied by the chemical decomposition of the electrolyte. The metallic conductors through which the current enters and leaves the electrolyte are called electrodes. The electrode at high potential is called anode and the other at lower potential is called cathode. In fact, the passage of current through electrolytes is considered to take place through moving charged particles (ions) with positive ions moving towards the cathode, and negative ions moving towards the anode.
In Alternative Current (AC) those reactions are inhibited by the periodic inversion of the current direction leading to a balance of the positive and negative current over the time.
The present invention proposes a solution that provides an optimized electrical swivel adapted for offshore use, having a light and compact design, having a high efficiency and requiring low maintenance and where when using DC current, the problem of electrolysis is strongly limited.